Human users access systems either as mobile or non mobile users, and from varying points of a network. Any system that requires a user to be (a) authenticated and/or (b) authorized to use a specific computer resource can be described as a human user system.
By this definition many public web sites are not human user systems, and indeed many of these sites welcome automated web crawlers and search engine computers when they seek access.
Any VPN server, or online banking website, or medical record website however, usually requires a human user to go through an authentication/authorization check first—these are human user systems.
Today, human user systems are subjected to various security and fraud attacks, including botnet attacks which attempt to take over a server and steal data, including users' private information.